docs: README, dead-code cleanup, smoke fixes

- README with run instructions and key reference
- lift the crate-level dead_code allowance; scope it to the anim library surface
- render paths tolerate a city that is still generating (district/building ids
  without entries), found by driving the app through a pty
- status messages show in the status bar, not only inside the inspector
- buffered JSON export (26 s -> 0.4 s for a large city)

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

crates/city/src/screens/generating.rs

@@ -64,6 +64,7 @@ impl Generating {
     }
 
     /// Something that changes whenever generation advances; used by tests.
+    #[cfg(test)]
     pub fn progress_marker(&self) -> (usize, Option<(Stage, u32)>) {
         (
             self.done.len(),